1. Field of the Invention
The present invention relates to a light emitting device using a light emitting diode or the like as a main light source, being intended for general illumination and a projector, or the like.
2. Description of the Background Art
As a light emitting device used for general illumination and a projector, a combination of a concave mirror and a discharge lamp or a halogen lamp, in which a focal point of the concave mirror is positioned at a light emitting point of the lamp, is widely used. However, the discharge lamp or the halogen lamp needs large electric power and has large heat discharge. Thus, a light emitting diode (LED, representing a light source that has a lesser amount of light and a lesser amount of heat discharge than the discharge lamp) has been proposed to be used as a light source of the light emitting device. In the light emitting device, in order to compensate for the disadvantage of the LED that the amount of light emission per one unit is smaller than that of the discharge lamp and the halogen lamp, a light emitting device having a plurality of LEDs is developed so as to emit a larger amount of light (for example, patent document 1: Japanese Laid-Open Patent Publication No. 2007-101732, FIG. 4 and FIG. 5).
As shown in FIG. 14, a light emitting device 1 according to patent document 1 includes two LEDs 2, and a concave mirror 4 having arranged thereon divided curved surfaces 3 which are obtained by cutting a surface of revolution, having a focal point, into two along a plane passing through the focal point, and by slightly isolating the divided curved surfaces 3 from each other such that the divided curved surfaces 3 have separated focal points Fx and Fy. The LEDs 2 are arranged at the focal points Fx and Fy, respectively, so as to face the light reflection surfaces 5 of their corresponding divided curved surfaces 3.
According to the light emitting device 1, the light emitted from each of the LEDs 2 is reflected on the corresponding light reflection surface 5. When the light reflection surface 5 constitutes a part of the paraboloid, the reflected light travels as parallel light, whereas when the light reflection surface 5 constitutes a part of an ellipsoid, the reflected light converges on a light converging point. Accordingly, it is possible to use light from a plurality of LEDs 2 more efficiently in the form of the parallel light, or by reducing unnecessary light (=stray light) that does not converge.
However, in the light emitting device 1 of patent document 1, as above described, a surface of revolution having a focal point is divided into a plurality of divided curved surfaces 3, and the divided curved surfaces 3 need to be arranged slightly distant from each other so as to have individual focal points, respectively. That is, a special concave mirror 4 is required and thus, it is impossible to use an ordinary concave mirror having a paraboloid (or an ellipsoid) light reflection surface. The light emitting device 1 has a problem of lack of versatility.
In addition, the above-described special concave mirror 4 has a problem of its manufacturability. That is, in the case of manufacturing the concave mirror 4 with glass, a thickness of the concave mirror 4 needs to be biased in accordance with shapes of the plurality of divided curved surfaces 3, which leads to deterioration in a yield of the material, and which causes difficulty in improving the accuracy of the shape. Moreover, even in the case of using aluminum, the above-described problems are caused, and it is substantially impossible to mold the concave mirror 4 by spinning. Moreover, in the case of using resin, a die for molding the concave mirror 4 will be of a complicated shape, and consequently, a cost for manufacturing the die is increased, and in addition, it will be difficult to improve the accuracy of the shape.
In addition, as above described, the concave mirror 4 has a plurality of focal points, and thus a portion of light, which is emitted from an LED 2 disposed at one focal point, is reflected on a divided curved surface 3 on a side opposite to the divided curved surface 3 facing the focal point, and is consequently converted to certain parallel light or converging light, which limits improvement in efficient use of light.